Polyester resins are derived from glycols and dicarboxylic acids by condensation reactions. In processing the reactants through polymerization, there are principally two stages involved, i.e., a first stage of transesterification and a second stage of polycondensation in which the reactants are converted to high polymeric condensation polyester resins. Under the thermal and catalytic conditions of the polymerization, there is a tendency for the polymer that is being formed to undergo decomposition. Polymer decomposition is evidenced by loss in molecular weight, liberation of carboxyl end groups giving rise to an increase in acid value, coloration of the polymer melt and deterioration of mechanical properties.
The effects of interesterification catalysts upon the instability of polyester resins have been reported upon, for example, in H. Zimmerman, Faserforschung u. textiltechnik, 13, 481 (1962). Catalysts not only promote transesterification and polycondensation but also induce various modes of thermal decomposition of the polyester resin. More specifically, in the conversion of dimethyl terephthalate to bis 2-hydroxyethyl terephthalate, the reaction is promoted through the equilibrating action of catalysts such as zinc acetate dihydrate, calcium acetate hydrate, manganese acetate tetrahydrate, and others. Conversion to maximum viscosity polyester is efficiently accomplished by catalysts such as lead (II) acetate and oxide, titanates and, in particular, antimony compounds such as antimony (III) oxide and acetate. A combination of catalysts is usually indispensible to commercially accetable polymerization conditions. However, with such catalytic combinations, polymer decomposition results as evidenced by viscosity drop, coloration, etc., as mentioned above.
Thus, it has been proposed to incorporate additives both during and after polymerization to improve the stability of polyester resins. For example, U.S. Pat. No. 3,676,393 is directed to stabilizer additives of simple organic phosphonates, phosphonites and phosphinites in polyester compositions. Other additives such as organic phosphates and ammonium phosphates have been employed. Generally, there are a number of difficulties associated with employment of the known additives. For example, in the addition of rather simple phosphonates or phosphonites, there is a tendency for such derivatives to distill out under polymerization conditions essential to high polymeric resin formation. In addition, some of the known derivatives tend to be extremely susceptible to unsatisfactory rearrangement reactions, and interfere with polymerization, or undergo thermal elimination reactions. With respect to known arylphosphonites or phosphonates, while they possess a certain amount of volatility, the aryl groups tend to be cleaved by acids and are detrimental to polymer color. Furthermore, when the molecular weight of known simple phosphorus derivatives is increased to reduce volatility, the phosphorus content of the derivatives is reduced thereby reducing their effectiveness. Suffice it to say that the prior art approaches to reduce the instability of polyester resins have not been completely satisfactory.